1. Field of the Invention
The present invention relates in general to a power transmission system and a hybrid drive system for a motor vehicle, and more particularly to techniques for controlling a vehicle drive force upon shifting of a power transmitting device from a non-drive position such as a neutral position "N" to a drive position such as a forward drive position "D", and electrically disconnecting a power transmitting path between a drive power source and drive wheels of the vehicle.
2. Discussion of the Related Art
JP-A-7-67208 discloses an example of a known hybrid drive system of a motor vehicle, which is equipped with (a) a drive power source including an engine operated by combustion of a fuel, and a motor/generator, and (b) a power transmitting device such as an automatic transmission interposed between the drive power source and drive wheels of the vehicle. Generally, the power transmitting device has non-drive positions (non-drive state) such as a neutral position "N" and a parking position "P" in which power is not transmitted therethrough from the drive power source to the drive wheels, and drive positions (drive state) such as a forward drive position "D" and a reverse position "R" in which the power is transmitted therethrough. The power transmitting device is electrically or mechanically shifted from one of those operating positions to another, for instance, from one of the non-drive positions to one of the drive positions, by operation of manually operated selecting means such as a shift lever.
There is also proposed a hybrid drive system of a motor vehicle, as disclosed in Japanese Patent Application No. 7-294148 (not published at the time the present invention was made) filed in the name of the assignee of the present application. Such a hybrid drive system includes (a) a synthesizing/distributing mechanism which includes a first rotary element connected to the engine, a second rotary element connected to the motor/generator and a third rotary element connected to an output member, and which is adapted to mechanically synthesize and distribute forces in relation to those rotary elements, (b) electrically neutralizing means for placing the hybrid drive system in an electrically neutral state or mode in which the motor/generator is placed in a non-load state while the above-indicated second rotary element is permitted to freely rotate, for disconnecting a power transmitting path between the engine and the above-indicated output member, when one of the non-drive positions of the power transmitting device is selected by the manually operated selecting means, and (c) start-up control means operated when the power transmitting device is shifted from the non-drive state to the drive state, for gradually increasing a reaction torque of the motor/generator from zero for transmitting the power from the engine to the output member through the synthesizing/distributing mechanism, and wherein the power transmitting device is disposed between the output member of the synthesizing/distributing mechanism and the drive wheels of the motor vehicle. U.S. Pat. No. 5,258,651 discloses a planetary gear device used as the synthesizing/distributing mechanism.
Commonly, such a hybrid drive system is not equipped with a fluid-operated torque converter as provided on an ordinary motor vehicle having an engine as the drive power source and an automatic transmission. Therefore, the hybrid drive system may suffer from a considerable shifting shock or an excessive amount of drive line load, due to an abrupt change in the vehicle drive force upon operation of the manually operated selecting means to shift the power transmitting device from a non-drive state to a drive state. If, for example, the drive position is selected by the selecting means while the engine or the motor/generator is operating with the accelerator pedal being depressed when the power transmitting device is placed in the non-drive state, an excessive load may act on the power transmitting device and other components in the drive line, or the power transmitting device may be subject to a large shifting shock. The hybrid drive system having the synthesizing/distributing mechanism tends to suffer from such an excessive load or shifting shock when the engine is operated at a relatively high speed with the power transmitting device placed in the non-drive state, if the characteristic relating to an increase or rise of the reaction torque of the motor/generator is permanently fixed.
In the hybrid drive system including the synthesizing/distributing mechanism and the electrically neutralizing means as described above, the power transmitting path between the engine and the drive wheels of the vehicle is electrically disconnected. Described more specifically, the operation of the manually operated selecting means to a non-drive position (non-drive state) to electrically neutralize the hybrid drive system is detected by a suitable detector, and a controller commands the electrically neutralizing means to establish the electrically neutral state of the hybrid drive system, in response to an electric signal generated by the detector. To provide mechanical fail-safe means for mechanically disconnecting the power transmitting path upon operation of the selecting means to the non-drive state, a suitable actuator should be provided to mechanically disconnect the power transmitting path between the engine and the drive wheels, upon generation of the electric signal from the detector when the shift lever is operated to the non-drive state. Thus, the mechanical fail-safe means requires such an exclusive actuator and complicate control of the actuator, leading to an increase in the cost of manufacture of the hybrid drive system.
The second power disconnecting means may use suitable clutch means such as frictional coupling clutches, and positive or claw clutches, for mechanically disconnecting the power transmitting power between the drive power source and the vehicle drive wheel. Where the positive clutches are used, each of these clutches is operated by a suitable releasing member such as a hub sleeve which is mechanically connected to the manually operated selecting means such as a shift lever, through connecting means such as a push-pull cable or a linkage. The frictional coupling clutches may be of a hydraulically operated type or a spring type such a diaphragm spring type. Where the frictional coupling clutches of the spring type are used, each of these clutches is operated by a releasing member such as a release fork connected to the manually operated selecting means through suitable connecting means as described above. Where the frictional coupling clutches of the hydraulically operated type are used, each of these clutches is operated by switching of a hydraulic circuit by a manual shift valve which is mechanically connected to the manually operated selecting means through suitable connecting means.
The power source may includes an engine and an electric motor. In this case, the power transmitting system may further comprise an electrically controlled torque converter which includes the electric motor and a planetary gear mechanism. In this instance, the second power disconnecting means may include clutches for disconnecting the power transmitting path between the drive power source and the drive wheels when the manually operated selecting means is operated to the non-drive or neutral state.
While the first power disconnecting means may be adapted to be operated to electrically disconnect the power transmitting path when the manually operated selecting means is operated to the non-drive state, the first power disconnecting means need not be so adapted, since the power transmitting path is mechanically disconnected by the second power disconnecting means when the selecting means is operated to the non-drive state.
Where the power drive source includes an engine and an electric motor, the first power disconnecting means may include means for establishing an electrically neutral state of the power transmitting system in which the electric motor is in a non-load condition with a rotor shaft thereof being freely rotatable, to electrically disconnect the power transmitting path.